__________________________________________________________ The U.S. Department of Energy Computer Incident Advisory Capability ___ __ __ _ ___ / | /_\ / \___ __|__ / \ \___ __________________________________________________________ INFORMATION BULLETIN Microsoft - Improve TCP Initial Sequence Number Randomness October 26, 1999 17:00 GMT Number K-006 ______________________________________________________________________________ PROBLEM: The algorithm which generates the initial sequence numbers (ISN) of a TCP packet is not as strong as it should be. Microsoft has discovered that "although not perfectly predictable, certain sequence numbers were used more frequently than others." This has the effect of reducing the resistance to certain types of attacks, such as IP spoofing and session hijacking. PLATFORM: Microsoft Windows NT 4.0 Workstation and Server. DAMAGE: If an ISN were to be predicted, it increases the risk for certain types of attacks, such as IP address spoofing and session hijacking. SOLUTION: Apply the patch. ______________________________________________________________________________ VULNERABILITY Risk is medium. The patch should be loaded on any Windows NT ASSESSMENT: Server which supplies information to the Internet or contains important internal data. ______________________________________________________________________________ [Start Microsoft Advisory] Microsoft Security Bulletin (MS99-046) -------------------------------------- Patch Available to Improve TCP Initial Sequence Number Randomness Originally Posted: October 22, 1999 Summary ======= Microsoft has released a patch that significantly improves the randomness of the TCP initial sequence numbers (ISNs) generated by the TCP/IP stack in Microsoft(r) Windows NT(r) 4.0. Improving the randomness of ISNs eliminates a class of potential attacks against Windows NT 4.0 systems. Frequently asked questions regarding this vulnerability can be found at http://www.microsoft.com/security/bulletins/MS99-046faq.asp. Issue ===== The ISNs used in TCP/IP sessions should be as random as possible in order to prevent attacks such as IP address spoofing and session hijacking. This patch improves the randomness of the Windows NT 4.0 TCP/IP ISN generation, providing 15 bits of entropy. Affected Software Versions ========================== - Microsoft Windows NT 4.0 Workstation - Microsoft Windows NT 4.0 Server - Microsoft Windows NT 4.0 Server, Enterprise Edition - Microsoft Windows NT 4.0 Server, Terminal Server Edition Patch Availability ================== - x86: http://download.microsoft.com/download/winntsrv40/patch/ 4.0.1381.7014/nt4/en-us/q243835.exe - Alpha: http://download.microsoft.com/download/winntsrv40/patch/ 4.0.1381.7014/alpha/en-us/q243835.exe NOTE: Line breaks have been inserted into the above URLs for readability. More Information ================ Please see the following references for more information related to this issue. - Microsoft Security Bulletin MS99-046: Frequently Asked Questions, http://www.microsoft.com/security/bulletins/MS99-046faq.asp. - Microsoft Knowledge Base (KB) article Q243835, How to Prevent Predictable TCP/IP Initial Sequence Numbers, http://support.microsoft.com/support/kb/articles/q243/8/35.asp. (Note: It may take 24 hours from the original posting of this bulletin for this KB article to be visible.) - CERT Advisory CA-95.01, Topic: IP Spoofing Attacks and Hijacked Terminal Connections, http://www.cert.org/advisories/ CA-95.01.IP.spoofing.attacks.and.hijacked.terminal.connections.html. (Note: A line break has been inserted into the above URL for readability) - Microsoft Security Advisor web site, http://www.microsoft.com/security/default.asp. Obtaining Support on this Issue =============================== This is a fully supported patch. Information on contacting Microsoft Technical Support is available at http://support.microsoft.com/support/contact/default.asp. Acknowledgments =============== Microsoft acknowledges National Bank of Kuwait for bringing this issue to our attention. Revisions ========= - October 22, 1999: Bulletin Created. ------------------------------------------------------------------ THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. (c) 1999 Microsoft Corporation. All rights reserved. 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[End Microsoft Advisory] ______________________________________________________________________________ CIAC wishes to acknowledge the contributions of Microsoft Corporation for the information contained in this bulletin. ______________________________________________________________________________ CIAC, the Computer Incident Advisory Capability, is the computer security incident response team for the U.S. Department of Energy (DOE) and the emergency backup response team for the National Institutes of Health (NIH). CIAC is located at the Lawrence Livermore National Laboratory in Livermore, California. CIAC is also a founding member of FIRST, the Forum of Incident Response and Security Teams, a global organization established to foster cooperation and coordination among computer security teams worldwide. CIAC services are available to DOE, DOE contractors, and the NIH. 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